Genome-wide identification of NRAMP family genes in Populus trichocarpa and their roles in transport of heavy metals

The NRAMP (natural resistance-associated macrophage protein) gene family is a group of metal ion transmembrane transporters widely found in animals and plants. Poplar trees present fast growth properties ideal for investigating the absorption and transport mechanisms of heavy metals by woody plants, therefore suited for the phytoremediation of heavy metal-polluted soils. However, the mechanism of the NRAMP gene family that may be involved in the transport of heavy metals in poplar yet remains unknown. In this study, we identified ten PtrNRAMP genes that are here analyzed through their gene structure, phylogenetic relationship, transmembrane domain, and subcellular localization. Subsequently, PtrNRAMP genes are divided into three subclasses based on their phylogenetic relationships; meanwhile, evolutionary analysis revealed that PtrNRAMP genes were amplified by tandem or segmental duplications. All PtrNRAMP proteins presented a conserved NRAMP domain, and the results of rice protoplast subcellular localization indicated that most PtrNRAMP proteins are localized on the cell membrane and/or vacuole membrane. Tissue expression analyses showed that the PtrNRAMP genes are involved in poplar growth and development. Under different levels of heavy metal stress, PtrNRAMP1.1 and its paralogs are significantly induced at the transcription level, whereas the expression levels of PtrNRAMP2.0 did not change significantly. Furthermore, most of PtrNRAMPs exhibited the absorption activity of Cd2+, Fe2+, and Mn2+ in yeast cells: PtrNRAMP1.1 and PtrNRAMP1.3 being able to absorb and accumulate a large amount of Cd2+ and Fe2+, and PtrNRAMP3.1 displayed the unique capacity in Mn2+ absorption and transport in yeast cells. These results will provide new criteria for elucidating the biological functions of PtrNRAMP genes, especially their role in mediating the uptake and transport of heavy metals in poplar.